In recent years, as digital technology rapidly spreads, systems have emerged for distributing digitalized content (hereinafter simply referred to as content) either by recording media, such as optical discs, or over a network.
One characteristic of such content is that quality does not degrade despite duplication. In order to protect the copyright of such content, it is necessary to prevent the holder of the content from making unauthorized copies. Therefore, within the system for distributing content, a transmission device that transmits the content and a playback device that plays back the received content each confirm that the other device is a legitimate device that protects copyright. Only when such confirmation is successful does the transmission device transmit the content to the playback device. In this way, the devices that use the content can be restricted, thus allowing for protection of the copyright on the content.
Confirmation that the other device is legitimate makes use of authentication technology based on a public key cryptosystem. An example of authentication technology based on the public key cryptosystem in Patent Literature 1 is as follows.
The transmission device transmits a random number to the playback device. Next, the playback device uses a private key allocated to the playback device in order to generate a signed text by applying a digital signature to the received random number. The playback device then returns the signed text to the transmission device. Finally, the transmission device verifies the received signed text using the public key of the playback device.
In such authentication technology that uses a public key cryptosystem, it is assumed that the public key itself is valid. In order to perform authentication using a public key cryptosystem, an organization referred to as a Public Key Certificate Authority issues a public key certificate indicating that the public key corresponding to a device is legitimate. The public key certificate authority also issues a revocation list (also referred to as a Certificate Revocation List (CRL) or a Public Key Revocation List) listing information identifying the revoked public key certificates, in order to notify other devices that among the issued public key certificates, public key certificates have been revoked for a reason such as the period of validity having expired, the device to which the certificate was assigned having been operated maliciously, or the private key having been divulged.
The revocation list includes a plurality of pieces of revocation information. Each piece of revocation information includes a serial number identifying the revoked public key certificate.
In Patent Literature 1, a personal computer is composed of a main body and a drive unit that accesses a recording medium. A check unit included in the main body reads a public key certificate from a buffer memory and extracts an identifier of the drive unit from the read public key certificate. Next, the check unit determines whether the extracted identifier is included in a drive identifier list that includes revocation identifiers. When it is determined that the extracted identifier is included, the main body considers the drive unit to be unauthorized and suspends communication with the drive unit.